Sunday 1 April 2018

Flooding kills at least four as Cyclone Josie sweeps past Fiji.

Four people are known to have died and one is still missing on Viti Levu Island, after Cyclone Josie passed to the south of Fiji on 1 April 2018. A 55-year old man has been found dead in Lautoka, a woman in Ba and two people in Nadi, where a 19-year-old man was seen to be swept away by floodwaters and has not yet been located. The area around Nadi has been particularly badly hit, with most of the town affected by flooding. The area received very little warning of the storm, which formed overnight around 120 km to the west of Kadavu Island, then passed to the south of the islands.

Flooding in the town of Nadi on Viti Levuiti Island, Fiji, after Cyclone Josie passed to the south on 1 April 2018. 9 News Australia/Facebook.

Tropical storms are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

 The passage of Cyclone Gita till 6.00 am GMT on Sunday 1 April 2018  (thick line) with its predicted future path (thin line, circles represent the margin of error on the predictions). Colours indicate the strength of the storm. Tropical Storm Risk.

The low pressure above tropical storms causes water to rise there by ~1 cm for every millibar drop in pressure, leading to a storm surge that can overwhelm low-lying coastal areas, while at the same time the heat leads to high levels of evaporation from the sea - and subsequently high levels of rainfall. This can cause additional flooding on land, as well as landslides, which are are a common problem after severe weather events, as excess pore water pressure can overcome cohesion in soil and sediments, allowing them to flow like liquids. Approximately 90% of all landslides are caused by heavy rainfall.

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